It is becoming very popular for manufactures to adapt passive radiators as a means of providing bass performance in small-enclosure stereo systems products. Integration of passive radiators requires careful tuning to effectively produce the much needed bass missing in these small systems. A sampling of the currently available products in this market clearly indicates that the tuning requirements are not being met.

Passive radiators respond to frequencies, presented as internal air pressure from the active speakers, on each side of a resonance frequency. Therefore, tuning for optimum performance is significantly more demanding and generally not well supported by the typical audio systems-on-a-chip integrated circuits on the market.

What makes passive radiators unique is that the tuning requirements are below the response range of the system's active speakers. The active speakers produce less acoustic energy (audible output, air movement) as the frequencies become lower or their volume is lowered. A system with a passive radiator needs to be tuned such that the passive radiator is active at frequencies below the active speaker counterparts and when the volume, air pressure, is less than optimum (maximum).

Passive radiators assume the requirement of producing lower bass frequencies from the generally smaller diameter active speakers. The lower bass frequencies require more air movement to be equally loud therefore, passive radiators are typically larger and are also capable of greater motion; movement in and out.

In addition, the enclosure's internal acoustic frequency response falls off at 6 dB/octave while the passive radiator performance falls off at 18 dB/octave leading to a requirement to maintain audible bass (internal air pressure) at lower volume levels (lower air pressure from the active speakers).

Additional care must be taken to avoid issues at the resonant frequency of the passive radiator; the displacement of the passive radiator is at a maximum (low volume level) when the active speaker is at a minimum (high volume level). Under these conditions, there can be an audible drop in bass output and unwanted audible harmonic distortion if the system is not properly tuned.

A modern system-on-a-chip audio integrated circuit should provide a means for,

Easily setting low frequency performance without distortion; getting maximum air pressure from the active speakers to the passive radiator while avoiding distortion in either passive or active speakers.

A means to maintain the internal air pressure, and therefore bass performance, as the active speaker's volume, air pressure, is decreased from maximum.

Additional features that systems like this can benefit from,

Enhanced stereo separation; the small size of these products place the active speakers in very close proximity thereby diminishing any stereo effect in the source audio.

Compression compensation; typical audio source content is from portable devices where compressed audio files are utilized optimizing the limited memory available. The side effect of compressed audio is reduced high frequency content.

Dynamic range compensation; listening environments are typically less than optimal and background (environment) noise can be well above the dynamic range of the source making it difficult to hear some of the source content.

Additional features that would enhance the end-user experience,

Graphic Equalizer or Profiles: simple features that enable the end-user to have some control over the audio reproduction.

Platform apps: provide additional features and controls for the end-user to enhance their listening experience.